This invention relates to a method of defect detection in step-and-repeat alignment and exposure systems. Such systems are used for projection printing of a pattern from a reticle onto a semiconductor wafer.
In particular, step-and-repeat alignment and exposure systems print a circuit pattern appearing on a reticle onto a semiconductor wafer having a photosensitive coating. This is done by repeatedly projecting an image of the reticle onto the wafer using actinic light. Such projection systems are used for device fabrication having reductions of greater than 1, typically 10:0 and 5:1 wherein the reticle contains a single copy or multiple copies (full field printing) of the device pattern to be employed. While the reticles are designed, built and maintained to be defect free throughout their use, contamination is a significant problem.
Defect-free use is especially significant in step-and-repeat systems because any defect on the reticle will be printed at every chip site as the pattern is stepped over the wafer. Thus, a defect in the reticle will repeat at each chip site which not only reduces yield but can potentially render all chip sites unacceptable.
Various techniques have disclosed in the literature to minimize such defects. Reference is made to Levy, "Automated Equipment for 100% Inspection of Photomasks", Solid State Technology, Vol. 21, pp. 60-71, May 1978 and Singer, "Photomask and Reticle Defect Detection", Semiconductor International, April 1985, pp. 66-73 for a generalized discussion on the problem and potential known solutions in the technology.
Reference is made to U.S. Pat. No. 4,443,096 which is related to a precision step-and-repeat alignment and exposure system for printing circuit patterns on a wafer. A pair of apertured optical detectors are aligned with identical portions of the projected image of the reticle and scanned across the image of the reticle. Any difference in the electrical image of the two optical detectors indicates the presence of dirt or a flaw in the reticle.
The optical comparator technique as used in the '096 patent therefore detects defects in the reticle by electro-optically comparing the imagery of two adjacent chip sites on a chip array. However, in the case of a step-and-repeat alignment and exposure technique to print a circuit pattern, the printing on the reticle onto a semiconductor wafer having a photosensitive coating, given the repeated projection of the image on the reticle, any reticle defect will be repeated at each chip site. Thus, conventional comparative techniques for defect detection are not functional in a step-and-repeat wafer imaging system.
One technique of eliminating dirt particles from the reticle is to use a thin, transparent protective membrane which is placed over the mask at some distance from the mask surface. This membrane, known as a "pellicle" is employed to protect the mask and render any particle falling on its surface out of focus in the image plane. Pellicles as such are disclosed in U.S. Pat. No. 4,131,363. Pellicles, however, can protect only to a certain degree or a given offset. There will be defects which, if located on the pellicle, can affect the printed image on the wafer. Thus, while the use of pellicles is effective to a certain extent, it is still necessary to detect and/or eliminate defects of a certain size on the pellicle itself. This problem becomes more acute as device sizes decrease. Image control becomes tighter and the devices are increasingly vulnerable to the smaller pellicle defects. The trend is for reduced device sizes and the use of optical lithography techniques that employ pellicles. The optical comparator techniques discussed in the prior art, while generally used to detect defects on the mask, can also be used in some limited instances to determine defects on pellicles. One continuing problem is the need in prior art techniques to physically handle or contact delicate pellicles.
Levy, supra, discloses one form of defect detection and a KLA100 series device manufactured by KLA Instruments Corp. discloses another type of commercially available optical comparator system. The inventor of the invention described herein has explored various defect types associated with pellicle parameters used in step-and-repeat projection optical microlithography systems. Such are reported in SPIE, Vol. 470, "Optical Multilithography III Technology for the Next Decade", pp. 138-146 (March 1984). A number of defect types are explored vis-a-vis the sub-micron patterns which are produced in a repeating sequence over the entire substrate surface. As is apparent from the discussion therein and the prior art, since stepper reticles do not contain repeated patterns, conventional comparator techniques cannot be used for their inspection. Thus, a need exists to allow comparator technology to be applied for defect detection produced by defects appearing in or on the reticle or the pellicle.